What causes that a lens/sensor gets dirty?
The front element of a lens is the part exposed to the environment when shooting, and unless you use a protective filter above it, it will get dirty at some point.
Dust, liquid stains and fingerprints are the usual reasons of why a lens gets dirty. Unless you're doing something you shouldn't be doing with a lens, there may be other reasons, but those three are the most common.
In a sensor, the most common trouble is dust. Dust is everywhere; except in a sterile, air locked room. But you're not going to live or take pictures in a room like that, you will shoot everywhere and anywhere, and dust will be around.
So how does dust falls on a sensor?
Everytime you change a lens, you expose the mirror box and dust particles have a chance to get inside it. Once you turn on the camera, the shutter will open every time you take a picture, revealing the sensor and dust will have a clear path to attach to it. The problem with sensors is that they are electrical devices, therefore they generate static electricity, which attracts dust particles.
Unless you work with small apertures (f/16,22,32) you wont see if there is any dust on your sensor, however, you shouldn't just ignore the issue and let it keep going, the more dust your sensor has and the longer it has it, the more difficult it will be to remove it, since the dust will get electrically charged and practically bond with the filter.
Other sources of contamination are liquid stains if you attempted to clean the sensor with liquids or if you touched your sensor with your greasy fingers as I told you NOT TO.
There are a LOT of ways to clean a sensor and a lens, but depending on how skilled and experienced you are, you may end up making a worse mess than you already have.
The problem is that, if you do it wrong, you may practically ruin your lens or your sensor. How much it cost you one or both of them is what determines how big of a mistake you can do.
In lens cleaning, the most common options are liquid and tissue or a cloth. In sensor cleaning the main go to options are the wet method; which consists of custom made swabs and a special solution to clean the sensor or the dry method which consists of blowing air into the sensor.
Before going any further, I should clarify that you don't actually clean the sensor itself, but a low pass filter in front of the sensor. The sensor never gets touched, but the filter. It's common practice to refer to the sensor because that's what you basically look at when cleaning the filter. Back in the dawn of digital age, DSLRs didn't have a filter in front of the sensor and makers weren't aware of the dust problem, so sensors would end up getting REALLY dirty, and the solution they came up with was to put a filter in front of it along with a vibrating mechanism to dislodge any dust that fell on it.
The Sony Alpha DSLRs contain this vibrating mechanism which is helpful to remove dust, but for that stubborn dust that wont go away, there are options.
The key point
The most given advice when doing sensor cleaning is:
IF YOU DO NOT TRUST YOURSELF TO DO THIS, DO NOT DO IT, TAKE YOUR CAMERA TO AN AUTHORIZED SERVICE CENTER.
We will establish this as an axiom.
Possible problems of different cleaning methods
Liquid and tissue: If you use more liquid than necessary, you may spill it inside the lens and leave stains on the glass.
Other problem of this method is that you rub against the glass with a liquid, which in turn, affects the coatings the lens or filter may have on them. You may wear off the coatings if you use this method frequently or with liquids not meant for optics.
Soft cloth: Even though there are cloths specifically designed for lens cleaning, there is 1 big but to this method.
This method will work great once, maybe twice, but then it will become a danger for your lens and here's why:
You touch the cloth with your FINGERS, which in turn are full of SKIN OILS, and those end up in the cloth, so the next time you use it, you'll end up smearing the lens with them.
You may think "Well, Ill just put it in the washing machine and get rid of those skin oils".
WRONG, this will only make matters worse. Water has calcium, calcium is a type of crystal mineral that can scratch your lens.
Air blower: This method is safe but it's not a panacea (a solution for everything). It will get rid of dust in your lens, but not from smudges, finger prints or sticky dust. If you got a lousy blower, it may not even blow enough to remove dust.
Wet method: If done properly, this is a very effective method to remove dust and any other stuff that doesn't belong in the sensor. The problem is that, if done wrong, you can leave smears or actually move the sensor out of place if you press too hard.
Usually the wet method is done by people with enough confidence and skill when everything else failed. But this shouldn't be attempted by rookies. You should train first simulating the procedure on glass or other similar material that wont be ruined if you screw up.
The problem of the wet method is that you can't do it quickly, materials are somewhat expensive and requires a clean, steady place to do it. Not to mention time, patience and great hand control.
There is one issue to keep in mind: sensor cleaning liquid is flammable, so forget about taking it on a plane; high pressure and flammable liquids: not a good combination.
There are plenty of liquids out there that can be used to clean a sensor using the wet method, but there is only ONE that's allowed by Sony to clean the sensors on the Alpha DSLRs that wont affect the low pass filter protecting the sensor.
The liquid is: Eclipse E2 Solution
DO NOT USE ANY OTHER LIQUID BUT THAT ONE. If you use any other liquid, it that may corrode the filter and even the sensor.
Air blower: This is the default recommended option suggested by camera manufacturers to clean your sensor. But for it to work, you need a good enough blower, a flimsy one wont do much.
They recommend this method because you don't really touch the filter, therefore you can't do something which will ruin the camera.
The problem with this is that not all dust responds to it, some electrically charged or mold dust sticks to the sensor and needs physical contact to remove it.
WHATEVER YOU DO, DO NOT NOT NOT PLACE THE BLOWER'S NOZZLE RIGHT ON THE SENSOR AND DO NOT USE BULB (SHUTTER SPEED) TO CLEAN YOUR SENSOR. IF YOU USE BULB AND YOU TAKE YOUR FINGER OFF THE SHUTTER WHILE THE NOZZLE IS AGAINST THE SENSOR, THE SHUTTER CURTAINS WILL CLOSE AND CRASH AGAINST THE NOZZLE, EFFECTIVELY RUINING YOUR CAMERA FOR GOOD.
If you got a strong enough blower, it may even remove that dust, but if it doesn't
DO NOT NOT NOT NOT NOT NOT EVEN THINK OF USING CANNED AIR.
Canned air is great for cleaning computers, but computers don't have sensitive material that you may end up screwing up by THE TINY PARTICLES THROWN BY CANNED AIR.
Canned air has been proved to contain rubber particles and other garbage inside the can, and if you use this to clean your sensor, you will just pass them on to it. DO NOT USE CANNED AIR FOR SENSOR CLEANING.
It's just as bad as putting some scotch tape on the sensor and pulling it. Sure, you will remove a lot of dust and whatnot, but you ll leave glue on the filter. IT'S JUST PLAIN SILLY.
Sensor and lens cleaning is an easy process, the scary part is that if done wrong, it can ruin your sensor or lens. There are many ways to do something wrong and a few (if not only one) to do it right. Again, if you don't trust yourself enough to do this process, take/send it to your camera's manufacturer service centers or places that know what they are doing like Adorama and Calumet Photographic .
There are a lot of methods to clean a sensor and a lens, some are the best way, some are just nonsense and very risky. On this article I discussed the most common methods to clean a lens and a sensor and their possible problems.
On the following two articles, I will review two products Ive recently used to clean my lenses and my sensor that yielded perfect results.